1. Field of the Invention
The present invention relates generally to a mobile communication system, a communication control method, and a base station and a mobile station to be employed in the mobile communication system. More particularly, the invention relates to a mobile communication system, in which a vacant time containing no communication data is provided in one or more of communication frames to be transmitted from the base station to the mobile station and a process, such as quality measurement of radio link of a different frequency carrier, for example, is performed during the vacant period at the mobile station.
2. Description of the Related Art
In the mobile communication system, wide service area is realized by providing unit communication service regions called as cells and establishing a service area as aggregate of the cells. The cell is defined as a range to be covered by one base station. On the other hand, since each base station has to cover a large number of user signals and so forth, a plurality of frequency carriers useful for communication are provided. For example, in an arrangement of frequency shown in FIG. 8, frequency carriers 401 and 402 can be used simultaneously by the same base station.
When the mobile station during communication moves within the service area, transition is caused from the current cell and a new and adjacent cell to perform switching of radio links in order to switch the base stations for establishing connection. For example, in FIG. 9, there is a case to switch communication link from a link 601 to a link 611. Transitional process to be caused upon switching of the radio links to connect is referred to as “hand over”. Switching of the radio links will be caused even when the mobile station does not move. For example, when a quality of the link in connection is low, when a communication speed is desired to be varied but variation of the communication speed cannot be achieved by the link in current connection, or so forth, switching of links to connect can be caused between the radio links of the same base station having a plurality of carriers (between the links 601 and 603 in FIG. 9) or between the radio links of a plurality of the base stations in the case where a plurality of base stations have cells simultaneously covering a point where the mobile station presents.
On the other hand, when a frequency of the carrier used in communication in the radio link before switching and a frequency of the carrier used in communication in the radio link after switching, the hand over is particularly referred to as “different frequency hand over”. In order to realize different frequency hand over, it becomes necessary to perform measurement of link quality in order to select a frequency carrier having high radio link quality as object for switching.
Conventional, in the case where a station has a period not performing transmission and reception, such as time division multiple access (TDMA), utilizing the vacant slot period, there is a method, in which a radio frequency is switched from the radio frequency in current communication to a frequency measurement object for using quality measurement of the radio link of the different frequency carrier. In this method, since communication and measurement of the different frequency carrier are not performed simultaneously, it can be adapted by switching of the radio frequency and it is not necessary to separately provide a radio machine for measuring the different frequency carrier.
On the other hand, in a system based on continuous transmission, such as frequency-division multiple access (FDMA) or code division multiple access (CDMA), there is a method to use two or more of plurality of radio machines, and to use the radio machine not used in communication for quality measurement of the radio link of other frequency carrier. As a method for not using two or more radio machines in CDMA, there has been proposed a Slotted mode (M. Gustafsson et al., “Compressed Mode Techniques for Inter-Frequency Measurements in a Wide-band DS-CDMA System”, Proceedings of the 8th PIMRC, September, 1997: First Prior Art). This is a system to compress a signal in time direction and to provide a vacant time having no data signal with maintaining a diffusing band unchanged by lowering spreading ratio over a plurality of time or elevating coding ratio of error correction coding, namely performing so-called puncturing.
FIG. 10 is an illustration showing an example of the case where the vacant period is provided. In FIG. 10, S1 to Sk+1 represent time slots. Referring to FIG. 10, there is shown one example, in which a signal transmission speed is increased to be double in comparison with those in periods T1 and T3 for compressing the signal into half in a time axis by lowering a spreading ratio in a period T2 into half. In a vacant period T4 thus obtained, a radio frequency is switched from a frequency carrier currently used for communication to another different frequency carrier to measure quality of the carrier.
On the other hand, there is another technology, in which a channel prediction (detection of amplitude information and phase information) is performed precisely by using a plurality of pilot blocks inserted in a transmission signal and a signal to noise power ratio is lowered for obtaining a necessary reception quality (bit error ratio or so forth) in synchronous or coherent detection (“Cannel Estimation Scheme Using the Plurality Pilot Blocks for DS-CDMA Mobile station”, Shingaku Giho, The institute of Electronics. Information and Communication Engineers, Japan, August, 1996, pp 45 to 50).
On the other hand, there is a further method for controlling a transmission power in a reverse link in the mobile station by performing transmission with inserting the pilot signal in an information system string from the mobile station, measuring a quality (SIR: signal power to interference power ratio) of a reverse link by receiving the pilot signal in the base station, comparing the SIR with a target value and reporting a result of comparison to the mobile station.
Furthermore, the pilot signal is also used for controlling the transmission power of a forward link. Namely, in the mobile station, the pilot signal transmitted from the base station is received for measuring the quality of the forward link, the measured quality, i.e. SIR, is compared with the target value to report the comparison result to the base station. Then, according to the result of comparison, the transmission power in the forward link is controlled in the base station.
However, in case of the slotted mode set forth above, the information for controlling the transmission power of the reverse link cannot be transmitted during vacant period. Therefore, a problem is encountered in degradation of characteristics of the reverse link. This problem will be discussed with reference to FIG. 9. It is assumed that the mobile station 620 connected to the base station 600 is performing communication using the link 601 as the forward link and the link 602 as the reverse link.
Structure of the transmission signal on the link 601 is illustrated in FIGS. 11A to 11E, for example. As shown in FIG. 11A, there is a super frame consisted of m in number of frames Fl to Fm. Each of the frames Fi is segmented into time slots Sl to Sn with a given interval as shown in FIG. 11B. Respective of the time slots Sj are in three kinds of forms as illustrated in FIGS. 11C to 11E. In FIG. 1C, a pilot signal PL, a transmission power control signal TPC, a transmission rate information RI and transmission data D1 are arranged in sequential order from the leading end of the time slot.
In FIG. 11D, a pilot signal PL, a transmission rate information RI, transmission data D1, a transmission power control signal TPC and transmission data D2 are arranged in sequential order from the leading end of the time slot. In FIG. 11E, a transmission rate information RI, transmission data D1, a transmission power control signal TPC, transmission data D2 and a pilot signal PL are arranged in sequential order from the leading end of the time solt.
The pilot signal PL is a signal for synchronous detection and quality measurement as set forth above. The transmission power control signal TPC is a transmission power control information in the reverse link (e.g. reverse link 602). At this time, when the link 601 enters into a different frequency carrier measurement mode and thus enters into the vacant period T4 of FIG. 10, the transmission power control information of the reverse link 602 becomes not transmitted by the link 601, the transmission power of the mobile station 620 is offset from an appropriate level to cause degradation of the characteristics of the reverse link 602. This degradation becomes more significant at longer vacant period T4.
On the other hand, when reception using a plurality of pilot symbols is to be performed in the mobile station, reception using the former and later pilot symbols becomes impossible due to presence of the vacant period T4 to cause degradation of the reception quality. On the other hand, when transmission of the pilot signal from the base station is not effected, SIR measurement in the mobile station is not performed. As a result, the transmission power control of the forward link in the base station becomes impossible.